Process of casting alkali metals



1955 G. w. MATTSON PROCESS OF CASTING ALKALI METALS Filed Nov. 29, 1951 I/VVENTOR. GEORGE I "47 7 5017 BY W United States Patent PROCESS OF CASTING ALKALI METALS George W. Mattson, Baton Rouge, La., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware Application November 29, 1951, Serial No. 258,874

2 Claims. (Cl. 22200.1)

This invention relates to the forming of reactive metals, especially the alkali metals. More specifically, the invention relates to the continuous transformation of such metals from the liquid state to solid easily handled forms or shapes.

Heretofore, in providing an alkali metal, for example, sodium, in form suitable for relatively small consumption shipments, it has been customary to manually ladle the molten sodium into individual molds and to let solidify. The solidification of such bricks or ingots is accompanied by the appearance of a void or aperture in the brick, which necessitates pouring an additional quantity of sodium to provide a complete brick, or mechanically working and swagging additional material in this void. The deficiencies of the procedure are obvious, in that it inherently is wasteful effort and also requires manual handling of the relatively hazardous material being processed. Further, the surface of the metal is contaminated with reaction products owing to exposure to a gaseous atmosphere at elevated temperature.

The object of the present invention is to provide a method of casting or forming the alkali metals in a convenient and uniform solid form directly from a liquid state. A further object is to provide a continuous process for carrying out such a solidifying and forming system. Another object is to provide a process for forming uniform masses, that is, forms which are not a mechanical aggregation of several pourings in the same mold, but are the result of the cooling and forming of a single batch or supply of molten metal corresponding to the final solid form. An additional object is to accomplish a casting of the alkali metal without surface contamination.

The above as well as other objects of the present invention will be more readily understood from the following description of some of its exemplifications, reference being made to the accompanying drawing wherein:

Figure 1 schematically shows a system for forming an alkali metal in accordance with the present invention; and,

Figures 2 and 3 are sectional views of pumps suitable for use in the system of Figure 1.

It has been discovered that these reactive metals are very readily formed in bars of any convenient length and of uniform cross section by adding a small amount of a lubricant liquid while the metal is in a liquid state, and then forcing the thus formed mixture through a tube or channel and cooling and solidifying during such passage. The lubricant used may be any material which is substantially inert to the alkali metal at the temperature of the metal on being forced into the tube where it is solidified. In addition, the lubricant should be a liquid at the temperature of operation, or at least of a semi-liquid consistency.

In passage through the channel or die, heat is removed, by any convenient means such as a coolant in a surrounding jacket, at a rate sufiicient to solidify the metal and to slightly subcool it below the solidification temperature. In carrying out this solidification, it has been found that concurrently with the solidification the lubricant is, by the solidification process alone, forced from mixture with the metal and deposited at the interface, that is, between the surface of the bar and the channel.

The manner of carrying out the process will be more readily understood from the following explanation of the figure, described with reference to the processing of sodium. In Figure 1 of the drawing there is diagram- 'ice matically shown a container 10 which is used as a source of molten sodium. This container preferably has a thermally insulated or otherwise suitable wall construction to keep the sodium at or above its melting point. A storage temperature of about 110 C. or higher has been found quite effective. Leading from container 10 is a conduit 12 which runs to a die 14, with a surrounding jacket 24 for a coolant medium. Control or block off of sodium fiow is provided by valve 16 in conduit 12. Also included in the conduit is a pump 20, which may be of the positive displacement or piston type connected to draw the molten sodium from the container 10 and force it towards the die 14. Moderate pressures are used to force the metal through the system. These pressures are of the order of about 100 pounds per square inch gauge pressure and higher, usually about 200 to 300 pounds pressure. The sodium leaves the die as a solid, homogeneous bar and is cut in desired lengths by a cutting wire or similar means.

On the discharge or pressure side of the pump 20 is a pressure gauge 22, conveniently used to follow the operation of the apparatus, the die 14, and a heat absorber shown as a cooling jacket 24 through which a coolant such as kerosene or ethylene glycol, or even water, circulates. The heat absorber is arranged to cool the supplied liquid sodium sufiiciently for it to definitely solidify and preferably reach a temperature of about 80 to C. If desired, the coolant can be selected so that it has a boiling point at about the temperature desired for the discharging sodium, and the coolant recirculation can then be in the form of a reflux condenser connected to the cooling jacket. Isopropyl alcohol makes an effective coolant for this purpose.

Alternatively, the heat absorbing unit can take the form of a gaseous heat exchanger or air radiator and can also be heavily finned to provide the desired cooling even though the unit is relatively small. In addition, currents of air or other gas can be impelled over the external surfaces of the radiator to also improve the cooling.

It is advantageous to surround the conduit by a tubular jacket through which a hot liquid may be circulated to pre-heat the assembly when starting up operation. This will prevent solidification of the hot sodium as it moves through the conduit. Preferably all parts of the conduit including the container 10 and those conduit portions into which the pump 20 is fitted, are so jacketed. However this feature need not be utilized, as for example when the entire conduit is made very short and in such close heat transfer relation to the container 10 as to be kept hot by this container. Where jacketing is used, it can also be emptied after operations are under way, and thereby provide an air cell insulating construction for the conduit to reduce thermal losses.

A feature of the present invention is the introduction of lubricant into the sodium while it is still molten. For this purpose, the lubricant can be fed into the container 10 or any portion of conduit 12 between the container and the heat absorber 24. As shown in the figure, this can be at a T connection 30 in the conduit 12 on the suction side of pump 20, at a T connection 32 on the pressure or discharge side of the pump, or at a separate suction intake line 34 for the pump. The T connection 32 requires the use of separate impelling means such as a pump 36 to force the lubricant into the conduit against the operating pressure maintained by main pump 20 on its pressure or discharge side. T 30 need not have such a pump if the lubricant is provided under some pressure or at a hydrostatic head sufiicient to overcome whatever pressure may exist on the suction side of main pump 20.

The rate at which lubricant is supplied can be controlled as by means of adjustable valves such as valve 40, in the lubricant supply lines, by mechanically interconnecting the lubricant and sodium supply pumps, or the like.

Effective lubricants for use in connection with the present invention are liquid paraffin hydrocarbons such as highly refined olefin-free mineral oil, or kerosene, organo-substituted polysiloxanes in which the organo groups are linked by carbon-to-silicon bonds (these compounds are commonly called silicone resins or oils) such as dimethyl polysiloxane, diethyl polysiloxane and phenyl ethyl polysiloxane. However, in general, any material which is a liquid at the forming and discharge temperature and which is substantially inert to the sodium which it is in contact with is suitable.

With lubricant added in the amount of about one milliliter per pound of sodium, it has been found that sodium is readily processed at a rapid rate and with any desired diameter, at a pressure of less than 300 gauge pounds per square inch. Solidification is also effected with smaller proportions of lubricant but higher pressures are needed until when the lubricant is reduced to about 0.01 milliliter per pound of sodium, a pressure of as much as 1000 pounds per square inch is insuflicient. A minimum lubricant ratio for good operation is about 0.05 milliliter per pound of sodium. However, larger proportions of lubricant, even higher than one milliliter per pound of metal can also be used. Such high proportions do not appreciably affect the pressure that is required, but are frequently used to assure an adequate lubricant film on the product shape, especially with the metals which are more reactive than sodium.

An additional efiect of the lubricant addition is the reduction or complete elimination of pumping difficulties such as chattering of reciprocating parts, sticking of valves, start-up troubles, etc. Furthermore, the lower pressure possible with the lubricant enables the use of less sturdy and less expensive conduit components and increases the safety with which the system can be operated.

Although the lubricant is introduced into the molten metal and appears to become well distributed through it, it is surprisingly found that in the final shapes the lubricant is only on the outer surface. It appears to be thoroughly squeezed out from the interior of the metallic mass during the solidification. This also results in the formation of a very desirable protective film over the discharged bar that preserves it for a, relatively long period of time against attack by atmosphere moisture so that careful protection of the solid metal is not needed and it is readily placed in containers, sealed, etc., or used in any desired treatment without undue complication. If necessary, the protective film can be simply removed, as by washing the extruded metal with a volatile solvent such as dry ether, or by merely wiping the film from the surface.

It should also be pointed out that the desirable features of the present invention are only obtained when the lubricant is added to the molten metal. No feasible method has been found for accomplishing the desired benefits other than by this method. Thus, attempts to operate without the addition of a lubricant to the liquid metal results in an extremely high pressure resistance of well over 1000 pounds per square inch, which overloads the pump and conduits. This high pressure resistance is encountered even if the die is of perfectly uniform cross section, that is, when there is no size reduction in going through the die. Apparently it is the lubricant introduced by the present invention into the interior of the liquid metal that is responsible for the above desirable features.

The invention has widest usage in the preparation of sodium bars, because of the cheapness and extensive market for this alkali metal. It will be understood that the process is useful in the casting of the other alkali metals, that is lithium, potassium and cesium. These metals all exhibit similar characteristics with respect to ductility, reactivity, and melting at moderate temperature levels. Owing to the variation in melting points, the operating temperatures will vary. In all cases, it is desirable that the metal be sub-cooled slightly below the solidification temperature by the time it is discharged from the die. The preferred amount of subcooling is not more than about 20 C. below the solidification temperature of the alkali metal being processed. Thus, in the case of sodium, it is preferred to discharge the product at a temperature of 80 to about 85 C. If the discharge temperature is appreciably below this level, the pressure requirements are increased. On the other hand, if the discharge temperature is allowed to approach the melting point, very close control of the operation is required and in addition the product shapes are easily deformable.

The die is usually of circular cross section so that round bars are formed. However, differently shaped cross sections may be used when desired, such as, for example, rectangular or square cross sections whereby bars of a shape more suitable for packaging than round bars are produced. As a general rule, it is preferred that the die have a uniform sized cross section throughout its length, The use of a die accomplishing a size reduction is not precluded, but such area reduction increases the work and pressure requirements in the sys-.

tern and therefore tends to defeat to some extent the advantages of the process. The benefits are however still retained in such instances but the pressure requirements are increased due to the work required for size reduction.

The length of the die is afifected by the size of the cross section of the bar, that is, the length of the die is necessarily increased for an increase in the cross sectional area, if a comparable linear rate of discharge is to be maintained. As a general guide, about to pounds of sodium is cast per square foot of internal die area. A convenient size die is one of circular cross section having a diameter of from about 2 to 4 inches.

Figure 2 illustrates one practical form of pump that can be used in the system described in connection with Figure 1. Here the pump is of the positive displacement type having a body 50 internally of which is provided an operating chamber 52 in which a plunger 54 is arranged to reciprocate in any convenient manner. Chamber 52 communicates through two check valves 56, 56 connected in series, with an intake opening 58. Another series-connected pair of check valves 60 also establish communication from operating chamber 52 to an outlet passageway 62.

The check valves, shown as of the ball and seat type, are so arranged that withdrawal of plunger 54 from chamber 52 closes valves 60, 60 and draws liquid in through intake 58 and valves 56, 56 which do not prevent fiow in this direction. When plunger 54 is brought back deeper into chamber 52, is positively displaces the liquid therein, causing valves 56, 56 to close and forces the liquid under considerable pressure to move through valves 60, 60 and out by way of the discharge outlet 62. For use with liquid sodium, plunger 54 should have a relatively close fit with a cylindrical guide. To assure the absence of leaks a packing gland can be provided on the external side of the closely fitting guide, and asbestos packing, such as a chevron type unit, impregnated with graphite and/or oil can be used in this gland. Metallic packing is also suitable. However, in normal practice no sodium finds its way past the plunger and into the packing.

Figure 3 shows a modified form of pump according to the present invention. Here the pump body 150, generally similar to that shown in Figure 2, has in addition to the positive displacement features of the former pump an additional intake 159 provided with a separate check valve 157 that supplies a one way connection from intake 159 into the communication path between the two sodium intake check valves 156, 156. In the operation of the pump of Figure 3, an intake stroke by the pump plunger draws in sodium through sodium intake 158, and also draws up lubricant through line 159. These materials are mixed within the pump and discharged together by the positive displacement stroke. Control of the lubricant addition with the pump can be readily effected either by suitably restricting the passageway provided by lubricant check 157, or by a fixed or adjustable constriction or valve in line 159.

Although two specific forms of pumps are shown above, other pumps can also be used in the practice of the present invention. Thus centrifugal pumps that can provide the desired pressure are suitable. In addition positive displacement pumps of different types, such as gear pumps, can also be used. Instead of ball types of check valves, movable flap types can be substituted, and if desired, only a single check valve can be used in the intake and a single check valve in the outlet lines. However, in actual practice it sometimes appears that individual valves tend to stick in the open position. When two such valves are used in series, as shown in Figures 2 and 3 for example, simultaneous sticking of both valves practically never occurs.

For convenience in using the system of Figure 1, particularly to provide a controlled feed of lubricant by way of connection 30 at the suction side of pump 20, it is GI desirable to use an impelling pump, instead of merely relying on a hydrostatic feeding head. Any of the above types of pumps can be used for this purpose, the standard gear type pump being an extremely simple and very practical example.

As many apparently widely difierent embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What I claim is:

1. The process of continuously casting an alkali metal comprising adding a non-reactive liquid lubricant to the liquid alkali metal then forcing through a straight, cooled, die of uniform cross section, solidifying and sub-cooling the alkali metal therein to a temperature of not more than about 20 C. below the melting temperature of the alkali metal, the lubricant being in proportions of not less than one milliliter per pound of alkali metal.

2. The process of continuously casting sodium bars protected from atmospheric corrosion by a continuous hydrocarbon film, comprising adding a liquid paraflinic hydrocarbon lubricant to liquid sodium in proportions of not less than one milliliter per pound of sodium, then cooling and solidifying the sodium as a cylindrical bar of uniform cross section having a diameter of from 2 to 4 inches, the solidification being carried out by feeding the sodium and lubricant at a pressure of not over 300 pounds per square inch through a cooled straight conduit of uniform cross section and cooling therein to a temperature of to C., the feed rate being at from to pounds of sodium per square foot of cooling surface per hour.

References Cited in the file of this patent UNITED STATES PATENTS 1,438,951 Elrod Dec. 19, 1922 1,466,125 Faupel Aug. 28, 1923 1,800,938 Hedly Apr. 14, 1931 2,195,809 Betterton et a1. Apr. 2, 1940 2,376,518 Spence May 22, 1945 2,531,290 Narrow Nov. 21, 1950 2,540,523 Horn Feb. 6, 1951 

1. THE PROCESS OF CONTINUOUSLY CASTING AN ALKALI METAL COMPRISING ADDING A NON-REACTIVE LIQUID LUBRICANT TO THE LIQUID ALKALI METAL THEN FORCING THROUGH A STRAIGHT, COOLED, DIE OF UNIFORM CROSS SECTION, SOLIFYING AND SUB-COOLING THE ALKALI METAL THEREIN TO A TEMPERATURE OF NOT MORE THAN ABOUT 20* C. BELOW THE MELTING TEMPERTURE OF THE ALKALI METAL, THE LUBRICANT BEING IN PROPORTIONS OF NOT LESS THAN ONE MILLLILITER PER POUND OF ALKALI METAL. 